Filtering electrical power can be an important aspect of power distribution systems at many levels, from commercial AC power distribution down to the DC power rails within an instrument that performs sensitive tasks. There are many sources of electrical noise found in modern electronics, including line noise, RF noise from high frequency circuits, and digital noise from computer subsystems. Problems arise when noise-sensitive components must run on power systems already polluted by noisy sub-systems. While inductor-capacitor filters are effective both for sheltering sensitive components and isolating noisy ones in many applications, they are not practical for high levels of broadband low frequency noise, such as is typical of modern DC electric cooling fans.
Filters can take a variety of forms, some of the simplest being the RC and LC filters shown in FIGS. 1A and 1B respectively. Passive filters are extremely common, and are used in both signal and power circuitry. For example, the low-pass filters shown in the FIG. 1A and FIG. 1B could be used to attenuate AC components present on a DC power rail, or to separate low and high frequency AC signals. In many cases, the action of passive networks is reversible, meaning that they have a similar filter behavior if turned end-for-end. Consequently, such networks, if used to remove noise, can do so in both directions, thus protecting the load from the source, and the source from the load.
Well-designed LC filters are superb both as signal extraction or noise rejection circuits for frequencies from the tens of kilohertz into the megahertz. However, below a kilohertz, components for LC filters and capacitors for RC filters become progressively larger as frequency further decreases. The broadband performance of such low frequency filter components can also be sub-optimal, requiring multiple stages of filtering for different frequency ranges. Active filters such as that shown in FIG. 2 answer some of these problems for signal filter systems, allowing physically small circuits to have corner frequencies of a Hertz or less, while retaining correct behavior into the hundreds of kilohertz or low megahertz in many cases. However, active filters such as this do not pass power from the input to the output, so they are not power filtering devices.
In order to filter low frequency noise from DC power rails in an active manner, transistor-based filters are effective, although far less studied than operational amplifier-based circuits. A basic transistor filter is shown in FIG. 3. This circuit is also known as a capacitance multiplier. The transistor Q passes current to any load attached to Vout via its emitter, a very low impedance connection. Since the base-emitter voltage (Vbe) of a bipolar junction transistor (BJT) varies as the logarithm of the collector current, Vout varies only slightly for significant changes in load current. (At a room temperature of 300 Kelvin, a 10-fold increase in collector current corresponds to an increase in Vbe of approximately 60 mV.) In contrast, current is delivered to this circuit via the collector of Q, a high impedance terminal. Provided it remains above the saturation value of Q for a given current flow, the collector-base voltage (Vcb) of the transistor can vary over the full range specified for the device, with little impact on Vbe. (There is a small coupling between the two through the Early effect, but this is negligible in this circuit.) The important consequence is that the voltage appearing at the emitter of Q, is primarily determined by the voltage at its base, which in this circuit equals the voltage appearing across capacitor C. This voltage is in turn a filtered version of input Vin, fluctuations in voltage above the corner frequency of the RC network being attenuated. Finally, because of the near constant Vbe value of Q, fluctuations in Vin above the RC corner frequency are also attenuated at Vout,
Transistor Q acts as a voltage follower, or a current amplifier in this filter circuit. While the voltage characteristics appearing at Vout are very similar to those at the base of Q, the current capability at these two nodes is very different. The current passing out the emitter of a typical BJT (when operating in its linear region) are typically hundreds of times that into its base. Because of this, the current capacity of R and C can be small (i.e., large R-value and small C-value), and thus their physical sizes can be small for a given RC time constant or filtering capability. Since the voltage drop across Q (Vce) is in general around a volt for this circuit, Q does not dissipate large amounts of power, and can thus also be small despite a large current capacity. Consequently, these three components can make a very effective low-pass filter for low frequency noise, with much smaller components than the equivalent LC or RC power filter that passes the same current flow.
While the circuit of FIG. 3 protects the load from noise in a power supply, the reverse is not true. The power supply is not protected from noise developed in the load. Load-induced voltage changes are caused by rapid changes in current demand. Such current spikes or surges would propagate directly back through transistor Q into the power supply through Vin. This in turn would cause a voltage change across the power supply commensurate with its output impedance, in a similar manner as would occur if the filter were omitted from the circuit.
What is needed is a transistor-based filter that protects the power supply from noise developed in the load.